NASA Believes Astronauts Fixed Leak Outside Space Station [VIDEO]

After nearly five hours walking in space Saturday morning, two astronauts replaced an external coolant pump on the International Space Station, in an effort to fix an ammonia leak.

National Aeronautics and Space Administration astronauts Thomas Marshburn and Christopher Cassidy began their spacewalk at 8:44 a.m. Saturday, according to a report by the Christian Science Monitor.

The two successfully replaced a 60-pound pump box that NASA had pinpointed as the source of the leaking ammonia coolant, though the duo found no evidence of the frozen ammonia flakes that had originally led them to suspect the pump box, nor any evidence of damage to the box itself.

By 1:20 p.m. Saturday, Cassidy and Marshburn finished up with the spacewalk and were headed back to the airlock. They saw no sign of leaks coming from the new pump.

NASA engineers continued to pressure check the system, in order to be certain the new pump was working properly.

The spacewalk was quickly plotted out after ISS crew members alerted Mission Control about the leak on Thursday when they spotted "snowflakes" of frozen ammonia floating near the pump box. NASA indicated that it had already known of a slow ammonia leak, but the rate suddenly increased to about 5 pounds of lost coolant per day.

"We're happy, we're very happy [with the spacewalk]," said Joel Montalbano, Deputy ISS Program Manager, in a press conference in Houston after the spacewalk. He added that "we didn't see any obvious signs of leaks," but testing would continue on the station's cooling system in the coming days and weeks.

NASA's space station program manager Mike Suffredini said it's a mystery as to why the leak erupted on Thursday. One possibility is a micrometeorite strike.

The space station's electronic equipment is cooled by ammonia streaming through the plumbing, according to the Associated Press. There are eight such power channels and the seven others were operating normally.

If it turns out the old pump wasn't faulty, the leak will become another crew's responsibility to find and fix, since Mashburn and Mission Commander Chris Hadfield of Canada are scheduled to head back to Earth on Monday.